Every year between May and July, one of the most dramatic events in nature unfolds beneath the waves off South Africa's east coast. Almost nobody on land knows it is happening. It is called the Sardine Run, and it involves more biomass than the wildebeest migration across the Serengeti.

The Greatest Shoal on Earth

Each winter, cold water wells up along the Agulhas Bank at the southern tip of Africa and pushes north along the KwaZulu-Natal and Eastern Cape coastline. South African sardines follow it in extraordinary numbers. The shoals they form are often more than 7km long, 1.5km wide and 30m deep - dense enough to be visible from satellite and dark enough to be tracked from spotter aircraft flying above. Scientists at BioGraphic estimate that in terms of total biomass, the run rivals East Africa's great wildebeest migration, yet it takes place entirely underwater and receives only a fraction of the attention.

The Feeding Frenzy

The sardines do not travel alone for long. As many as 18,000 common dolphins work in coordinated packs, blowing streams of bubbles to herd pockets of fish upward into compressed, spinning "bait balls" near the surface. Once a bait ball forms, the spectacle erupts. Sharks drive in from below. Cape gannets fold their wings and plunge from heights of up to 20m, hitting the water at confirmed speeds of up to 86km/h (53mph). Fur seals wheel through the edges. Then, rising from below, a Bryde's whale - typically 12 to 14 metres long - lunges through the entire ball with its mouth open, swallowing thousands of fish in one pass. Each bait ball is gone in under 20 minutes.

An Ecological Trap

For decades, the reason the sardines make the run puzzled scientists. A landmark 2021 study in Science Advances offered a sobering answer: the run may be an ecological trap. The sardines follow a pulse of cold upwelling water northward - an initially favorable cue - but end up stranded in warm subtropical conditions that push beyond their physiological limits. The sardines that make the journey face higher mortality and lower reproductive success than those that stay on the south coast. The researchers proposed the migration may be a relic behavior from a previous glacial period, when the cold-water corridor extended much further north. The ocean has warmed. The instinct remains.

A Run That May Be Fading

At its peak in the early 2000s, the total sardine biomass involved was estimated at around four million tonnes. By the mid-2020s that figure had fallen below one million tonnes, with warmer ocean temperatures and fishing pressure both implicated. In some years the sardines fail to arrive at all. Researchers and dive operators who have tracked the run for decades describe it as less predictable than it once was - a warning sign for one of the ocean's most spectacular annual events.

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